13 SCDA8C 1 PAGE 1 BCD 07/23/ SCDA TIME SHARING SCHEDULING ALGORITHM 07/22/ T. HASTINGS AND R. DALEY SCDA0002 SCDA0003 MINOR MODIFICATIONS BY G. SCHROEDER WHEN NEW SCDA0004 I/O PACKAGE INSTALLED...SUMMER,1965. MINOR CHANGES FOR NEW COMMAND PROCESSOR SCDA0005 SCDA0006 SPRING P.R. BOS SCDA0007 SCDA0008 THE SCHEDULING ALGORITHM PERFORMS THE FOLLOWING FUNCTIONS SCDA0009 SCDA DETERMINES WHICH USER IS TO RUN NEXT 2. DETERMINES WHEN NEXT USER IS TO RUN 3. DETERMINES HOW LONG NEXT USER IS TO RUN 4. CHARGES USERS FOR SWAPPING AND RUNNING TIME 5. KEEPS TRACK OF THE STATUS OF EACH USER SCDA0011 SCDA0012 SCDA0013 SCDA0014 SCDA0015 CTSS Time sharing scheduling algorithm, preamble

14 In order to optimize the response time to a user's command or program, the supervisor uses a multi-level scheduling algorithm. The basis of the algorithm is the assignment of each program as it enters working or waiting command status to an nth level priority queue. Programs are initially entered at a level which is a function of the program size (i.e. at present, programs of less than 4k words enter at level 2 and longer ones enter at level 3). There are currently 9 levels (0-8). The process starts with the supervisor operating the program which is first in the queue at the lowest occupied level, L. the program executes for a time limit = P.L quanta; a quantum of time is one half second. If the program has not finished (left working status) by the end of the time limit, it is placed at the end of the next higher level queue. The program at the head of the lowest occupied level is then brought in. If program P enters the system at a lower level than the program currently running, and if the current program P1 has run at least as long as P is allotted, then P1 will be returned to the head of its queue and P will be run. CTSS Time sharing scheduling algorithm, 1969

15 Les procès définis par Salzer dans sa thèse, 1966

16 Structure générale par Salzer dans sa thèse, 1966

17 A computer system is a vehicle in which various tasks or processes are executed. In all computer systems, at least two primitive process control functions exist. These are: 1. The ability to create or introduce new processes. We will call this the process creation primitive. 2. The ability to forceably halt the execution of a process. This ability rests in some force or power outside the process (possibly in another process. We will call this the process destruction primitive. 3. The ability for a process to declare that it has finished and ought to be terminated. We will call this the suicide primitive. In his PhD, Salzer proposed to add 4 primitives: 1. The block primitive which includes suicide. 2. The wake up primitive. 3. The reschedule primitive (originally named restart by Saltzer). 4. The stop primitive (originally named quit by Saltzer). These four primitives make up what Saltzer calls the Process Exchange. The Process Wait and Notify (PWN) facility offered its users four entry points: addevent, delevent, wait, and notify. Addevent allowed a process to allocate an entry and to thread the entry onto a particular list. Delevent allowed a process to unthread itself from a list and deallocate its entry. Wait allowed a process to check that it was still on a given list. If the process was still on the list wait called block. If not, wait returned. Notify allowed a process to pick up an entire list, call wakeup for each process on the list and unthread the entries from the list. Procès dans la thèse de Rappaport, 1968

18 Structure générale par Rappaport dans sa thèse, 1968

19 This MTB proposes that the scheduler allow the grouping of processes into work classes and provide each work class with a guaranteed percentage of available cpu time. Conceptually each work class will be assigned a virtual processor [ ] The actual algorithm used to enforce the proper sharing of the cpu resource will be as follows: Imagine the existence of a system virtual clock which increments as virtual time is used by non-idle processes. Imagine also that each work class has a store of credits (in units of microseconds) which is continually growing at a rate proportional to the speed of the virtual clock multiplied by the fraction of cpu resources which the work class is to receive. Suppose further that the store of credits for the work class is decremented as members actually consume virtual cpu time. Clearly it is undesirable to allow credits to build up indefinitely for a work class with no processes ready, so a maximum value is set on the number of credits which can be accumulated. In addition the value is restricted from ever becoming negative. The algorithm for chosing the next work class from which to choose a process to which to award eligibility may then be as simple as choosing that work class which has accumulated the maximum number of credits. Multics Workclass Scheduler, 1975

20 In , Bob Mullen added features to the scheduler to provide more efficient support for daemons driving physical devices such as printers. allow precise tuning of workloads for competitive benchmarks. The version of the scheduler is called the Deadline Scheduler. The deadline scheduler used the workclass structures to implement a wholly different scheduler in which neither the FB-n nor the percentage parameters were used. Considering that most people do not understand the FB-n algorithm, the top level view was that the scheduler could be operated in either percent mode or deadline mode. Using some workclasses with virtual deadlines along with a few with realtime deadlines was a convenient way to tune benchmarks with response time requirements which varied for different user scripts. Multics deadline scheduler, 1976

21 The traffic control subsystem is implemented almost entirely by one large ALM program named pxss. Various entries into pxss are used by other ring-0 procedures to change the state of the current process or to send a wakeup signal to some other process. Most of pxss runs in a wired environment with interrupts masked, in order to protect critical data bases (most notably the wired segment tc_data). MDD-19, 1986

22 Without a theory of computing systems to fall back on,designing of such complex systems becomes an art, rather than a science, in which it is impossible to prove the degree to which working solutions to problems are in any sense optimum solutions. In much the same way as authors write books, large computer systems go through several drafts before they begin to take shape. In the absence of a theory one can only cope with the complexity of the situation by proceeding in an orderly fashion to first produce an initial working model of the desired system. This part of the work represents the major effort of the design and implementation project. Once having arrived at this benchmark, many of the problems may then be seen in a clearer light and revisions to the working model are implemented much more quickly than were the original modules. As to the development of a theory, one gets the impression that it will be a long time in coming. Conclusion de la thèse de Rappaport, 1968

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